Theoretical insight into the acidity and cooperativity effect of the LLM-105∙(HNO₃)₂ system

In order to reveal the nature of the acidity in explosive product, the acidities and cooperativity effects from the intermolecular H-bonding interactions in the LLM-105∙∙∙(HNO₃)₂ ternary systems were investigated at the B3LYP/6-311 +  + G** and M062X/6-311 +  + G** levels, with the integral equation formalism polarized continuum model (IEFPCM) based on the self-consistent-reaction-field. The results show that for the ternary systems, the intermolecular H-bonding interactions are stronger than those in the binary complexes, resulting in the lower [H⁺] concentrations and larger pK_a1 values upon the ternary-complex formations. However, there is no obvious correlation between the acidities and cooperativity effects or APT charges of the H atoms involving the H-bonds. Surface electrostatic potential (ESP) and reduced density gradient are used to reveal the nature of the H-bond and acidity. Interestingly, the acidity is closely related to the ESPs of the H atom (V_S,H) involving the intermolecular H-bond, but not to the statistical quantities. This is mainly because both acidity and V_S,H are the local properties of system, whereas the cooperativity and statistical quantity of ESP are the global property.